I. Field of the Invention
This invention relates to processes for separating ash particles from regenerated adsorbent particles, such as activated carbon.
II. Description of Related Art
Powdered adsorbent materials, such as activated carbon, coke fines, diatomaceous earth, fly ash and the like, are used in wastewater treatment in a variety of ways. For instance, such materials are added to biological treatment systems to enhance performance. Powdered activated carbon is most commonly used for this purpose and in some processes is mixed with biological solids in an aeration basin. Examples of such processes are described in U. S. Pat. Nos. 3,904,518 and 4,069,148.
Once the adsorptive properties of the activated carbon are expended, it must be regenerated before it can be reused in the treatment process.
One method for regenerating carbon involves wet oxidation of the mixture of biological solids and spent carbon. Wet air oxidation at elevated temperatures and pressures destroys the volatile portion of the biological solids and oxidizes the organic substances adsorbed on the surface of the powdered carbon to restore its adsorptive capacity. The resulting regenerated carbon is recycled as part of an aqueous slurry to the treatment process
Another method for regenerating carbon involves flame incineration under a controlled atmosphere, such as in a multiple hearth furnace. The hot thermally regenerated carbon and biomass residue mixture is cooled by introduction into a water quench tank. The resulting aqueous slurry of regenerated carbon is recycled to the treatment process.
The recovered aqueous slurry from either regeneration process consists primarily of reactivated carbon particles and inorganic ash particles removed from the wastewater by the carbon and formed during the regeneration process. The continued recycle of this ash along with the recovered carbon to the treatment process eventually leads to an undesirable accumulation of ash in the wastewater treatment system. Thus, there is a need to remove a portion of the ash from the regenerated carbon stream to prevent a detrimental build up of ash in the wastewater treatment system.
Pradt U.S. Pat. No. 3,808,126 discloses removing inert solids from the bottom of a wet oxidation reactor via a separate pipe and combining the inert solids stream with the reactor outflow at a point downstream in the system.
Burant et al. Canadian Pat. No. 1,073,365 discloses removing inert ash from a powdered activated carbon by classification of inert solids in a wet air oxidation reactor during carbon regeneration. The heavier inert solids are removed from the bottom of the reactor while a regenerated carbon slurry is removed from the top of the reactor and returned to the treatment system.
Pradt et al. U.S. Pat. No. 3,876,536 discloses separating ash from a regenerated carbon with a centrifical device which makes use of the difference in the specific gravity between the ash and the regenerated carbon.
Armold et al. U.S. Pat. No. 4,541,933 discloses separating ash from a wet oxidized activated sludge/powdered carbon mixture by using a plurality of hydrocyclones to concentrate the ash and then collecting the ash concentrate on a screen.
Japanese patent application No. 96713/81 discloses diluting a regenerated carbon/ash mixture with from two to ten volumes of water, and adding a dispersing agent if the total hardness is higher than 100, to suspend the ash particles and precipitate the carbon particles. The precipitated carbon is recycled to the treatment process. A cationic flocculent is added to the water suspension containing ash particles to precipitate the ash.
Sykes et al. U.S. Pat. No. 4,555,329 discloses separating inorganic gangues from coal particles in a coal refuse slurry by adding low molecular weight anionic vinyl polymers to disperse the slurry and then adding high molecular weight anionic vinyl polymers to flocculate and settle the coal particles.
All these prior processes have certain disadvantages. Classification of inert ash in a wet oxidation reactor requires additional piping and removal of the ash slurry at system pressure is extremely abrasive to control valves. Centrifical devices are costly to install and operate. Diluting the slurry from the regeneration process produces increased volumes of low strength wastewater containing inert ash which must be treated before discharge.